I don't expect synthetic fleece linings to provide much warmth. Shearling works because it creates lots of little air pockets that trap your body heat, much like down does. And wool has the remarkable property of feeling warm even when wet, which is great in shoes because they tend to trap moisture, especially if made of water-resistant or waterproof materials. I find polyester fleece makes my feet feel kind of clammy compared to wool. I have a pair of fleece-lined "winter sneakers" (below) but I don't enjoy wearing them if it gets below, say, -5°C or so. They're also on the small side now, so I can't really layer properly in them anymore.
When it's really cold like it has been the past couple weeks (daytime temps from -10 to -20°C), I've been wearing a silk liner sock to move moisture away from the foot, plus a wool boot sock, inside a shearling-lined boot or a Thinsulate boot with an aftermarket shearling insole (both in Finds). IME multiple layers inside roomy boots is the only thing that works for the depths of winter. (Old timers will tell you to size up in boots so you can get that pocket of warm air around your foot.)
